This section is intended to provide a background or context to the invention recited in the claims. The description herein may include concepts that could be pursued, but are not necessarily ones that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, what is described in this section is not prior art to the description and claims in this application and is not admitted to be prior art by inclusion in this section.
Lenses, and increasingly metalenses (lenses used in metasurface-based flat optics), are an integral part of a large number of optical systems, including microscopes, cameras, telescopes, spectrometers, and so on. In metasurface-based flat optics, metasurfaces are thin films with sub-wavelength (less than the wavelength of the light employed) thickness containing sub-wavelength in-plane features (meta-atoms) that are used to realize a desirable functionality by local modification of the interaction between the thin film and the incident electromagnetic fields. In recent years, such structures have attracted significant attention due to their potential to provide excellent control of properties of transmitted or reflected fields, such as directivity, polarization and orbital angular momentum, with low-profile conformal devices.
Light is electromagnetic radiation within a certain portion of the electromagnetic spectrum. The wavelength of visible light ranges from roughly 400 nm to roughly 700 nm. Monochromatic light consists of a single wavelength, and monochromatic lenses work for the light with the desired single wavelength. Problematically, metasurfaces using conventional phase-shifter elements for monochromatic lenses require high-aspect-ratio fabrication techniques not always compatible with standard semiconductor fabrication processes. Achromatic light consists of multiple wavelengths, and achromatic lenses can transmit multiple colors. Problematically, chromatic aberrations are a significant technical barrier that precludes the use of metasurface-based flat optics in the most demanding optical systems. Correction of chromatic aberrations has been addressed by various approaches, such as spatial multiplexing, multi-layer stacking, hybrid integration, multi-material dispersion compensation, and independent control of phase and group delay. Most of these approaches have achieved achromatic focusing. However, this conventionally comes at a cost of fabrication complexity, performance decline or limitations, such as limited lens size and numerical aperture, and polarization dependence.